In recent years optical sensors have become widely used in various technologies to make many different sorts of measurements. Many optical sensors, such as strain sensors and fiber optic gyroscopes, operate by making “time of flight measurements.” That is, such optical sensors measure the amount of time it takes for light to travel a particular optical path length, such as down the length of an optical fiber or around a coil of optical fiber. However, depending on the particular type of optical sensor, the measurement being made may not be of the total amount of time it takes for the light to travel the given path, but rather of the time difference it takes different portions of the light to travel the optical path.
For example, Optical Time Domain Reflectometers (OTDRs) are often used to determine the presence, as well as the locations, of various features along an optical fiber, such as optical components and deformations (e.g., cracks). OTDRs perform such measurements by emitting a pulse of light down the optical fiber and measuring how much time passes before reflections of the light return. As the light propagates down the optical fiber, some of the features cause a portion of the light to be reflected, while allowing another portion of the light to pass. The portion of light that passes through the feature will eventually be reflected back towards the OTDR, such as by the end of the optical fiber. The result is that the two portions of light travel different optical path lengths, and thus require different amounts of time to return to the OTDR. By measuring and comparing the “time of flight” for each of the portions of light, the OTDR can be used to determine not only the length of the optical fiber but also the locations of the features along the optical fiber that reflect the light.
In a similar manner, fiber optic gyroscopes (FOGs) use time of flight measurements to detect rotation by essentially comparing the time it takes different portions of light to travel in opposite directions around a coil of optical fiber. In FOGs, the time of flight difference is not necessarily measured by time per se, but by interference patterns caused by the two portions of light as they are captured by a photo-detector.
The resolution, and thus the sensitivity, of such optical sensors is limited by the sensitivity of the particular device or process being used to detect the returning light and measure the time of flight difference. For example, the resolution of optical sensors using an OTDR is limited by the smallest time difference (i.e., the most closely spaced portions of light) that the OTDR is able to detect. Therefore, if two features along the optical fiber are within a very small distance (e.g, a few microns), the two portions of the light will return to the OTDR during a very small window of time (e.g., a few femtoseconds). If the OTDR is unable to distinguish both portions of the light, one or more features along the optical fiber may not be detected.
Likewise, in a FOG, if the photo-detection used is unable to detect subtle interference patterns, the FOG may not be able to detect extremely low rates of rotation. Often, the sensitivity of FOGs is increased by lengthening the optical fiber used. However, such a solution has the disadvantage that it increases the overall size of the device.
Accordingly, it is desirable to provide a method and system for adjusting the sensitivity of optical sensors. In addition, it is desirable to provide a method and system that increases the sensitivity of optical sensors while minimizing overall size. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.